Re: [PATCH v4 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

[Baolin Wang]

On 2025/6/25 17:31, Lorenzo Stoakes wrote:
> On Wed, Jun 25, 2025 at 04:52:03PM +0800, Baolin Wang wrote:
> > On 2025/6/25 16:37, Lorenzo Stoakes wrote:
> > > Yeah maybe the best way is to just have another tunable for this?
> > >
> > > /sys/kernel/mm/transparent_hugepage/disable_collapse perhaps?
> > >
> > > What do you think Hugh, Baolin?
> >
> > I think it's not necessary to find a way to disable madvise_collapse.
> > Essentially, it's a conflict between the semantics of madvise_collapse and
> > the '/sys/kernel/mm/transparent_hugepage/enabled' interface. We should reach
> > a consensus on the semantics first:
> >
> > Semantic 1: madv_collapse() should ignore any THP system settings, meaning
> > we need to update the 'never' semantics in
> > '/sys/kernel/mm/transparent_hugepage/enabled', which would only disable page
> > fault and khugepaged, not including madvise_collapse. If we agree on this,
> > then the 'never' for per-sized mTHP would have the same semantics, i.e.,
> > when I set 64K mTHP to 'always' and 2M mTHP to 'never', madvise_collapse
> > would still allow the collapse of 2M THP. We should document this clearly in
> > case users still want 64K mTHP from madvise_collapse.
>
> Right yeah, I mean this is in effect how things are now. So the task is
> documentation.
>
> > Semantic 2: madv_collapse() needs to respect THP system settings, which is
> > what my patch does. Never means never, and we would need to update the
> > documentation of madv_collapse() to make it clearer.
>
> Yes, and indeed this is the choice.
>
> I think, as David said, it comes down to whether we have a legit use case that
> truly relies on this.
>
> > > (One side note on PMD-sized MADV_COLLAPSE - this is basically completely
> > > useless for 64 KB page size arm64 systems where PMD's are 512 MB :)
> > >
> > > Thoughts Baolin?
> >
> > We should not collapse 512MB THP on 64K pagesize kernel. So seems
> > madv_collapse() can not work on 64K pagesize kernel.
>
> Well I don't think anything would prevent this now right? So MADV_COLLAPSE is
> pretty problematic on 64K pagesize kernels in general.

Yes, I don't mean it will prevent madvise_collapse(), just as you said 
that it could be problematic (it's horrible to try to collapse 512MB).

> Anyway that's maybe a problem for another time :)

Yeah, should consider mTHP-compatible MADV_COLLAPSE.